Coated grid for electron discharge tubes



June 12, 1951 I LE ROY APKER 2,556,864

COATED GRID FOR ELECTRON DISCHARGE TUBES Filed Sept. 27, 1950 Fig. I.

GALL/UM 0R r/n/ ru/vssriew Inventor": L,eRoy Apker',

Patented June 12, 1951 '(loATED GRID FOR ELECTRON DISCHARGE TUBES Le RoyApker, Niskayuna, N. Y., assignor to General Electric Company, acorporation of New York Application September 27, 1950, Serial No.187,098

1 Claim.

This invention relates to the control of contact potentials betweenmetals and, more particularly, to the control of contact potentialsbetween the electrodes of vacuum type electron discharge devices.

In the vacuum tube industry, it is important that the operatingcharacteristics of electronic tubes remain substantially constantthroughout the life of the tubes. Consequently, the contact potentialfield existing between the grid electrodes and the cathodes shouldremain unchanged. This contact potential field exists because of thediiference between the work functions used in the grids and in thecathodes. Since the grids and the cathodes perform, in some respects,substantially opposite functions, they are, by necessity, constructed ofdissimilar metals having widely separated work functions.

It is well known in the art that evaporation products from thermioniccathodes contaminate the grids and lower the work function thereof. Infact, in a new tube, the work function of the grid is generally loweredto a value near the work function of the cathode and, consequently, thecontact potential field between the two is much reduced. It may,moreover, be unstable. To provide a vacuum tube suitable for use, in themanufacturing process the tubes are generally subjected to an agingprocess which is continued until the work function of the grids isstabilized, and the contact potential between them and the cathodesremains substantially constant thereafter. Aging of a tube is, ofcourse, a lengthy and expensive operation.

Therefore, an object of my invention is to provide a new and improvedelectrode which will have a relatively constant work function when usedin an electron discharge device in which the heretofore mentionedevaporation products are present.

In the attainment of the foregoing objects, I provide a grid structurethe surface of which is in the liquid state at the operatingtemperatures of the tubes. Because a liquid surface is much more activein dissolving evaporation products than a solid surface is, the workfunction thereof is maintained more nearly constant. Therefore, thecontact potential field between the grids and the cathodes is also morenearly constant.

For further objects and advantages and for a better understanding of myinvention, attention is now directed to the following description and.accompanying drawings, and also to the appended claim. In the drawing,Fig. 1 is a sectional view of a portion of a grid electrode constructedaccording to this invention; and Fig. 2 is a conventional vacuum typeelectron discharge device employing this type of grid electrode.

Referring to Fig. 1, a metallic substrate is shown coated with a thinlayer of tin. Because the normal operating temperatures of grids in mostvacuum tubes is of the order of 300 C., which is above the melting pointof tin, a liquid surfaced grid structure may thus be realized. The useof tin as the outer layer of the grid structure has been shown, but anymetal having a low melting point and a low vapor pressure attemperatures well above the melting point is applicable. Another suchmetal is gallium. Both tin and gallium have boiling points near 2000 C.,which is, of course, an indication that they have a low vapor pressureat temperatures well below this value. These characteristics of lowmelting point and vapor pressure are generally necessary to prevent theliquid surface from evaporating during operation of the tube throughoutperiods of time of the order of thousands of hours, which is a normallife span of a vacuum tube. It is also desirable that the metal used asthe substrate have a high cohesive energy to decrease the tendency ofthe liquid coating to dissolve or diffuse away. One such metal istungsten.

Referring to Fig. 2, a tin coated metallic grid structure I is mountedon grid supports 2 and 3 which are enclosed in a hermetically sealedenvelope 4 which is mounted in a base 5. Also enclosed within envelope 4is a cathode structure 6 and an anode structure I. Terminals 8 extendingfrom base member 5 permit control of the current and voltage betweenelectrodes l, 2 and 3. By maintaining the outer portion of grid member lin the liquid state, the contact potential between grid l and cathode 6,which is of primary concern in the construction of vacuum tubes, may

be maintained constant without aging of the.

tube.

A liquid surfaced grid structure as described herein provides a constantcontact potential between the grid and the cathode throughout the lifeof the tube without an extended and consequently costly process ofaging. As a result, an electronic tube employing a grid of this type hasoperating characteristics which remain unchanged from the time ofinitial operation throughout the life of the tube. Contact potentialsdifferent from those found in ordinary tubes may be obtained, and platecurrents may be very small even when positive potentials of one or twovolts are applied to the grid. Normally, the low work function of anordinary grid causes plate currents to be measurable even at negativeapplied grid voltages of as much as 1 volt.

While this invention has been described by a particular embodimentthereof, it will be understood that those skilled in the art may makemany changes and modifications without departing from the true spiritand scope of this invention. Therefore, by the appended claim, I intendto cover all such changes and modifications which fall within the truespirit and scope of this invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Name Date Pipper May 12, 1936 Number

